Converting a three primary color input signal into four signals

ABSTRACT

A method of converting a three primary color input signal into a four primary color drive signal, includes defining three functions representing the first, second, and third drive signals as a function of the fourth drive signal, determining the intersection values of the fourth drive signal at a set of intersections of the three functions mutually, and of the three functions and a line defined by the fourth drive signal being equal to itself, determining boundary values of a valid range of the fourth drive signal in which all drive signals have valid values, determining calculated values of the first, second and third drive signals at the intersection values and the boundary values, calculating the maximum value (Vmax) or minimum value (Vmin) of the calculated values at the intersection, and selecting the intersection value at which the maximum value or minimum value is minimum or maximum, respectively.

The invention relates to a method of converting a three primary colorinput signal into four drive signals for driving four primary colors ofa multi-primary color display, to a system for converting a threeprimary color input signal into four drive signals, to a displayapparatus comprising this system, to a computer program product forconverting the three primary color input signal into four drive signals,to a camera comprising the system, and to a portable device comprisingthe camera.

U.S. Pat. No. 6,570,584 discloses an OLED display device with moresub-pixels than the conventional red, green, and blue sub-pixels. Theadditional sub-pixel may have a color selected outside the color gamutpossible with the usual red, green, and blue sub-pixels to obtain awider color gamut. Alternatively, the additional sub-pixel may have acolor inside the color gamut possible with the red, green, and bluesub-pixels to increase the efficiency and lifetime of the display. Thedisplay system uses a conventional RGB input signal to create anappropriate multi-color signal for driving all the sub-pixels.

A color data transformation circuit converts the input signal to valuesnecessary for controlling each of the sub-pixels. The transformationcircuit calculates the appropriate amounts of light from each of thethree available sub-pixel colors to reproduce the desired color at eachof the four sub-pixels using well known matrix transforms or lookuptables. The amount of light produced by a sub-pixel depends on anumerical value supplied to the sub-pixel. The numerical value is, forexample an eight or six bit value. In general, the three primary inputsignal can be transformed into the four drive values of the sub-pixelsin different ways.

A transformation circuit which performs matrix transformations has thedrawback that a lot of matrix multiplications are required. Either anexpensive and fast calculation circuit is required or an algorithm hasto be constructed to pre-calculate the majority of the computationallyintensive mathematical calculations to enable real time operation. Apre-calculation and a transformation circuit which uses look-up tableshave the drawback that the algorithm is pre-fixed to one of the possiblesolutions.

It is an object of the invention to provide a method of converting athree primary color input signal into four drive signals for drivingfour primary colors of a multi-primary color display which conversionhas a high flexibility in selecting one out of the possible solutions.

A first aspect of the invention provides a method as claimed in claim 1.A second aspect of the invention provides a system as claimed in claim7. A third aspect of the invention provides a display apparatus asclaimed in claim 8. A fourth aspect of the invention provides a computerprogram product as claimed in claim 12. A fifth aspect of the inventionprovides a camera as claimed in claim 14. A sixth aspect of theinvention provides a portable device as claimed in claim 15.Advantageous embodiments are defined in the dependent claims.

The method in accordance with the first aspect of the invention convertsa three-primary color input signal into four drive signals which drivefour primary colors of a multi-primary color display. The three-primarycolor input signal usually is a RGB signal, but any other signal whichdefines the drive signals of a usual display which has three differentlycolored sub-pixels may be used instead. The three-primary color inputsignal may be defined in the XYZ space. But, now, in the multi-primarycolor display which has four differently colored sub-pixels, four drivesignals have to be supplied to the four sub-pixels. Usually, themulti-primary color display has, per pixel, the three standard RGBsub-pixels and at least one further sub-pixel with a different color.The different color may be selected inside or outside the color gamutdefined by the RGB sub-pixels. The different color may be white.

The three-primary color input signal comprising a first, a second, and athird input signal. The method converts these input signals into afour-primary color drive signal comprising a first, second, third, andfourth drive signal. These drive signals drive the four primary colorsof the multi-primary color display. Although it is said that the drivesignals drive primary colors, in fact the colors of the four sub-pixelsneed not be primary colors as defined in the colorimetric theory.Whichever colors are selected for the sub-pixels does not matter, theyact like primary colors because they determine the colors to bedisplayed on the display.

The method defines three functions representing the first, second, andthird drive signal as a function of the fourth drive signal. In a linearsystem, these three functions are straight lines. In a non-linear colorspace the three functions may define bending lines. Next, values of thefourth drive signal are determined for intersections of: (i) the threefunctions mutually, and (ii) the three functions and a line defined bythe fourth drive signal being equal to itself. These values of thefourth drive signals at all of these intersections are further referredto as the intersection values. With respect to the intersections definedby (i) and (ii), only the intersection values are relevant of functionshaving opposite signs of their first derivative. Further, theintersections of the functions with the boundary lines are founddirectly by calculating the values of the first, second, and thirdfunctions at the boundary values of the fourth drive signal. Theboundary values of the fourth drive signal are found by determining therange of values of the fourth drive signal where all four drive signalshave valid values. The valid values are values which are allowed in thehardware or software implementation of the conversion algorithm. Forexample, in a digital implementation, valid values may the values in therange 0 to 255 if the signals are represented with 8 bit words. Thevalid values may be normalized to, for example only, a range from 0to 1. The determination of the valid range of the fourth drive signal iselucidated in detail in the not yet published European patentapplication 05102641.7.

The method calculates the associated first, second and third drivesignals at the intersection values of the fourth drive signal to obtaincalculated values. At each one of the intersection values, either themaximum value or the minimum value of the set of values comprising theintersection value and the calculated values at this intersection valueis determined. This set of values is further referred to as the valuesof interest. Finally, the intersection value is selected at which themaximum value is minimum, or at which the minimum value is maximum.Thus, now a particular one of the all possible combinations of the fourdrive signals fitting the three input signals is selected. In fact, thefreedom in the conversion from the three input signals to the fouroutput signals is used to optimize the conversion under a constraintwhich in this case is either (i) the minimal value of the maximum valuesof drive values at the relevant intersections, or (ii) the maximal valueof the minimum values of the drive values at the relevant intersections.

The determination of the maximum value of the set of values, at eachrelevant intersection value of the fourth drive signal, provides thehighest one of the four drive signals at the associated intersectionvalue. If the intersection value is selected at which this highest valueis minimal, this is the conversion which produces the lowest maximumvalue of the four drive values. Which is, for example, advantageous in adisplay in which a backlight is modulated and the drive voltages arescaled accordingly. The selection of the conversion which produces theminimal maximum value allows the backlighting to be dimmed maximally. Ifthe intersection value is selected at which the minimum value ismaximal, this is the conversion which produces the highest minimal valueof the four drive values. This is, for example, advantageous in adisplay in which a gamma at low levels is not very reproducible and lowdrive voltages should be prevented as much as possible.

In an embodiment in accordance with the invention as defined in claim 2,in a linear system, the three functions are lines and thus can easily bedetermined.

In an embodiment in accordance with the invention as defined in claim 3,the intersections of lines with a different sign of their respectivefirst derivative can be determined with a simple equation.

In an embodiment in accordance with the invention as defined in claim 4,if an intersection occurs for an intersection value of the fourth drivevalue which is outside the valid range, this intersection value isclipped to the nearest boundary value of the fourth drive value. Theboundary values of the fourth drive value are the extreme values of arange of valid values of the fourth drive value.

In an embodiment in accordance with the invention as defined in claim 5,the valid range of the fourth drive value is determined by determiningfor the first, second, and third drive signal a respective range of thefourth drive signal for which the first, second, or third drive signalhas valid values. The valid range of the fourth drive value is definedas a common range of the fourth drive signal wherein each one of thefirst, second, and third drive signal has valid values.

In an embodiment in accordance with the invention as defined in claim 6,the input signal is clipped if the common range of the fourth drivesignal is not realizable.

In the drawings:

FIG. 1 shows schematically a block diagram of a converter in accordancewith the invention.

FIG. 2 shows an example of graphs of the four drive values as a functionof the fourth drive value,

FIG. 3 shows a flowchart of an algorithm in accordance with theinvention,

FIG. 4 shows a block diagram of the application of the algorithm in amodulated backlight display,

FIGS. 5A to 5C show graphs for elucidating the application in themodulated backlight display, and

FIG. 6 shows graphs of the four drive values as a function of the fourthdrive value for an RGBW display.

FIG. 1 shows schematically a block diagram of a converter in accordancewith the invention. The converter comprises a signal processing circuit4, a valid range determining function 1, further referred to as therange function 1, a selection function 2, a display drive function 5,and a display 3. The signal processing circuit 4, the range function 1,and the selection function 2 may be realized by a suitably programmedprocessor or by dedicated hardware which processes a software algorithm.

The signal processing circuit 4 receives an image input signal IV andsupplies the primary color input signal C1, C2, C3. The range function 1receives the three input signals C1, C2, C3 and a coefficient matrix MCto supply the common range VS of a fourth drive value P4. The threeinput signal components C1, C2, C3 are commonly referred to as IS, andthis signal is usually the RGB signal, but may be another signaldefining the amount of light of the three sub-pixels per pixel of astandard three sub-pixel display. In accordance with the presentinvention, these three input signals IS have to be transformed into, ormapped on, drive signals P1 to P4 for the four sub-pixels 30 to 33 perpixel 34 of the four sub-pixel display 3. The drive signals P1 to P4 arecollectively also referred to as DS. The range function 1 uses threefunctions F1 to F3 (see FIG. 2) which each define one of three of thedrive values P1 to P3 as a function of the input signals IS and theremaining fourth drive value P4. The coefficient matrix MC is defined bythe color points of the 4 sub-pixels.

The operation of the range function 1 is detailed in FIG. 2 for thethree to four primary conversion when normalized drive values are used.

The selection function 2 receives the valid range VS, the input signalsIS, the coefficient matrix MC, and a selection criterion or constraintCON to supply the N drive signals DS via the display drive function 5 tothe sub-pixels 30 to 33 of the display 3. The display drive function 5may comprise a gamma function when the operations in front of thedisplay drive function 5 are performed in the linear light domain.

The present invention is directed to implementing a particularconstraint CON such that out of the possible mappings from the threeinput signals C1, C2, C3 to the four drive signals P1 to P4 an optimalone is selected. First the three functions F1=P1(P4), F2=P2(P4), andF3=P3(P4) representing the first, second, and third drive signal P1, P2,P3 as a function of the fourth drive signal P4 are determined. Then, theintersection values P4 i of the fourth drive signal P4 are determined ata set of intersections of: the three functions F1, F2, F3 mutually, andof the three functions F1, F2, F3 and the line F4 defined by the fourthdrive signal P4 being equal to itself. Only the intersection values P4 iof functions having opposite signs of their first derivative arerelevant. Now, the first, second and third drive signals P1, P2, P3 atthe intersection values P4 i of the fourth drive signal P4 arecalculated to obtain calculated values CV1, CV2, CV3. Further, theassociated first, second and third drive signals P1, P2, P3 arecalculated at the boundary values P4min, P4max of a valid range VR ofthe fourth drive signal P4. These sets of values are referred to as thevalues of interest (CV1, CV2, CV3, P4 i) which comprise the intersectionvalues P4 i and the boundary values P4min, P4max of the fourth drivesignal P4 and the associated calculated values CV1, CV2, CV3. For eachset a maximum value Vmax or minimum value Vmin of the values of interestCV1, CV2, CV3, P4 i at the associated value of the fourth drive value P4is determined. The associated value P4 is either one of the intersectionvalues or one of the maximum value Vmax or minimum value Vmin. Finally,the value of the fourth drive value P4 at which the maximum value Vmaxor minimum value Vmin is minimum or maximum, respectively, is selected.This algorithm is further elucidated with respect to FIGS. 2 and 3.

The signal processing circuit 4 may be replaced by a camera, the threeinput signals C1, C2, C3 are supplied by the camera, and the inputsignal IV is not present. The camera may also supply the four drivesignals P1, P2, P3, P4. Preferably the camera accepts user inputdefining whether it should supply the three input signals C1, C2, C3 orthe four drive signals P1, P2, P3, P4.

FIG. 2 shows an example of graphs of the four drive values as a functionof the fourth drive value. The fourth drive signal P4 is depicted alongthe horizontal axis, and the three drive signals P1 to P3 together withthe fourth drive signal P4 along the vertical axis. The three drivesignals P1 to P3 are defined as functions of the fourth drive signal P4:F1=P1(P4), F2=P2(P4), and F3=P3(P4). The fourth drive signal P4 is astraight line through the origin and has a first derivative which isone. The valid ranges of the four drive signals P1 to P4 are normalizedto the interval 0 to 1. The common range VS of the fourth drive signalP4 in which all the three drive signals P1 to P3 have values withintheir valid ranges extends from the value P4,min to P4,max, and includesthese border values.

Preferably, a linear light domain is selected wherein the functionsdefining the three drive signals P1 to P3 as a function of the fourthdrive signal P4 are defined by

$\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} = {\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} + {\begin{bmatrix}{k\; 1} \\{k\; 2} \\{k\; 3}\end{bmatrix} \times P\; 4}}$wherein P1 to P3 are the three drive signals, (P1′, P2′, P3′) aredefined by the input signal which usually is a RGB signal, and thecoefficients ki define a dependence between the color points of the 3primaries associated with the 3 drive values P1 to P3, and the primaryassociated with the fourth drive signal P4.

To further elucidate the relation between the elements of thesefunctions it is now shown how the above functions relate to the standardthree to four primary conversion. In a standard three to four primaryconversion, the drive signal DS, which comprises the drive signals P1 toP4, is transformed to the linear color space XYZ by the following matrixoperation.

$\begin{matrix}{\begin{bmatrix}{Cx} \\{Cy} \\{Cz}\end{bmatrix} = {\begin{bmatrix}{t\; 11} & {t\; 12} & {t\; 13} & {t\; 14} \\{t\; 21} & {t\; 22} & {t\; 23} & {t\; 24} \\{t\; 31} & {t\; 32} & {t\; 33} & {t\; 34}\end{bmatrix} \times \begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3} \\{P\; 4}\end{bmatrix}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The matrix with the coefficients tij defines the color coordinates ofthe four primaries of the four sub-pixels. The drive signals P1 to P4are unknowns which have to be determined by the multi-primaryconversion. This equation 1 cannot be solved immediately because thereare multiple possible solutions as a result of introducing the fourthprimary. The input signal components C1, C2, C3 are now replaced by Cx,Cy, Cz to indicate that they are defined in the linear XYZ color space.

The question solved in the present invention is how to make an algorithmthat efficiently deals with the freedom in conversion and which isefficient such that a real-time determination of the common range ispossible. A particular real-time selection out of these possibilitiesfor the drive values of the drive signals P1 to P4 is applied.

Equation 1 can be rewritten into:

$\begin{matrix}{{\begin{bmatrix}{Cx} \\{Cy} \\{Cz}\end{bmatrix} = {{\lbrack A\rbrack \times \begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix}} + {\begin{bmatrix}{t\; 14} \\{t\; 24} \\{t\; 34}\end{bmatrix} \times P\; 4}}}{A = \begin{bmatrix}{t\; 11} & {t\; 12} & {t\; 13} \\{t\; 21} & {t\; 22} & {t\; 23} \\{t\; 31} & {t\; 32} & {t\; 33}\end{bmatrix}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$wherein the matrix [A] is defined as the transforming matrix in thestandard three primary system. Multiplication of the terms of equation 2with the inverse matrix [A⁻¹] provides Equation 3.

$\begin{matrix}{\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} = {\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} + {\lbrack A^{- 1} \rbrack \times \begin{bmatrix}{t\; 14} \\{t\; 24} \\{t\; 34}\end{bmatrix} \times p\; 4}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

The vector [P1′ P2″ P3′] represents primary values obtained if thedisplay system only contains three primaries. Finally, Equation 3 isrewritten into Equation 4.

$\begin{matrix}{\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} = {\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} + {\begin{bmatrix}{k\; 1} \\{k\; 2} \\{k\; 3}\end{bmatrix} \times P\; 4}}} & {{Equation}\mspace{14mu} 4}\end{matrix}$

Thus, the driving signal of any three primaries P1 to P3 is expressed byEquation 4 as a function of the fourth primary P4. These linearfunctions define three lines in a 2 dimensional space defined by thefourth primary P4 and the values of the fourth primary P4 as isillustrated in FIG. 2. All values in FIG. 2 are normalized which meansthat the values of the four drive values P1 to P4 have to be within therange 0≦Pi≦1. From FIG. 2 it directly visually becomes clear what thecommon range VS of P4 is for which all the functions P1 to P3 havevalues which are in the valid range.

Basically, if no such common range VS exists, then the input color isoutside the four primary color gamut and thus cannot be correctlyreproduced. For such colors a clipping algorithm should be applied whichclips these colors to the gamut.

A scheme which calculates the common range P4,min to P4,max iselucidated in the non pre-published European patent application05102641.7.

In the example shown in FIG. 2, the boundary P4,min of the valid rangeVS is determined by the function F2 which has a higher value than 1 forvalues of P4 smaller than P4,min. The boundary P4,max of the valid rangeVS is determined by the function F3 which has a higher value than 1 forvalues of P4 larger than P4,max.

The intersection of the functions F1 and F4 occurs at the value P4 i 1of the fourth drive value P4. The intersection of the functions F1 andF3 occurs at the value P4 i 2 of the fourth drive value P4. Theintersection of the functions F3 and F4 occurs at the value P4 i 3 ofthe fourth drive value P4. The intersection of the functions F2 and F3occurs at the value P4 i 4 of the fourth drive value P4. Theintersection of the functions F2 and F4 occurs at the value P4 i 5 ofthe fourth drive value P4. The intersection of the functions F1 and F2is not shown.

The intersection at P4 i 3 and the intersection of the functions F1 andF2 are not relevant because the first derivative of the intersectingfunctions has the same sign. Also, the intersections P4 i 1 and P4 i 5are not relevant, because these intersections are outside the validrange VS. At each one of the other intersections P4 i 2, P4 i 4, and atthe border values P4min, P4max the values of the functions F1 to F3 aredetermined. In the example shown, only the values CV11, CV21, CV31 ofthe functions F1 to F3 at the intersection P4 i 1 and the values CV14,CV24, CV34 at the intersection P4 i 4 are shown. The value of thefunction F4 at the intersection P4 i 1 and P4 i 4 is equal to theintersection value.

Now it is checked at each intersection and border value which one of thefunctions F1 to F4 has the highest (or lowest) value. In the exampleshown, the highest value of the functions F1 to F4 is for the bordervalues and the intersection values within the valid range: P4min, P4 i2, P4 i 4, P4max, respectively: 1, CV22, LMAX, 1.

Next the intersection or border value is selected at which the highestvalue is minimal (or at which the lowest value is maximal). In theexample shown the minimal highest value of the functions at theintersection or border values are the values CV24 and CV34 of thefunctions F2 and F3, respectively, at the intersection value P4 i 4.These minimal highest values are indicated by LMAX.

FIG. 3 shows a flowchart of an algorithm in accordance with theinvention. In step S0 the variables i and u are set to zero. In step 1the variable j is made equal to the variable i+1. In step S2, the signof the coefficient k(i) is compared with the sign of coefficient k(j),wherein the coefficients k(i) and (kj) are the coefficients k1 to k3 inequation 4. If the signs are equal, the algorithm proceeds with step S10and increments the variable j by 1. In step S9 it is checked whether theincremented variable j is smaller than 4. If yes the algorithm proceedswith step S2 if no, in step S8, the variable i is incremented by 1 andin step S7 is checked whether the incremented variable i is smaller than4. If yes the algorithm proceeds with step S1, if no, the algorithmproceeds with step S11.

If in step S2 is detected that the signs are not equal, and thus thelines have opposite signs of the first derivative, in step S3 theintersection value P4 i of the two lines is determined by the equationP4i=(Pj′−Pi′)/(ki−kj)wherein the Pj′ and Pi′ are respective ones of the P1′ to P4′ ofequation 4 to which is added that the fourth drive signal (P4) which isequal to itself is defined byP4=P4′+k4*P4 wherein P4′=0 and k4=1.

In step S4 is checked whether the intersection value P4 i is smallerthan the upper limit P4max of the valid range and larger than the lowerlimit P4min of the valid range. If the intersection value P4 i is notwithin the valid range, the algorithm proceeds with step S10. If theintersection value P4 i is within the valid range, in step S5 its valueis stored as P4(u), and in step S6 the value of u is incremented by 1.

In step S11, the lower border value P4min is stored as P4(u), in stepS12 the value of u is incremented by 1, in step S13 the border valueP4max is stored as P4(u), and in step S14 the variable u is set to 1. Instep S15, for the actual value of u, the values P1 to P3 are calculatedby using equation 4 for the value of P4 stored for this value of u. Thestored value of P4 is indicated by P4(u) and is either one of theintersection values P4 i or one of the border values P4min, P4max. Alsothe stored value P4(u) itself is retrieved as the value for P4.

In step S16, the maximum value of the values P1 to P4 is stored as P4m(u). In step S17, the value of u is incremented by 1, and in step S20is checked whether u<size(P4). Wherein the size(P4) is a number which isthe sum of the number of intersection values P4 i and the two bordervalues P4min, P4max.

If yes, the values of P1 to P4 are calculated in step S15. In step S16,the maximum value P4 m(u) of the values P1 to P4 is determined andstored. In step S17 the variable u is incremented by 1, and thealgorithm proceeds with step S20. After calculation of all the maximumvalues, the outcome of the check in step S20 is no, and the in step S18,the minimum value P4opt of all the stored maximum values P4 m(u) isdetermined. The core of the algorithm now ends in step S19.

The values of the other drive values P1 to P3 can now be calculated bysubstituting this optimal value P4opt in the three functions F1 to F3.This minimum value P4opt (which is Pui4 in FIG. 2) defines the selectedmapping from the three primary color input signals Cx, Cy, Cz into thefour drive values P1 to P4, wherein the selection is performed in usinga particular constraint. The particular constraint is that, of the setof values of the functions F1 to F4 in all points of interest, the setof values is selected of which the maximum value is minimal. The pointsof interest comprise all intersection points P4 i of the functions F1 toF4 and the two border values P4min, P4max. Alternatively, the particularconstraint may be that, of the set of values in every point of interest,the minimum value is determined and that the point of interest isselected at which this minimal value is maximal.

If all the intersections are outside the valid range VS, p4opt will beequal to one of the boundary values p4min or p4max.

FIG. 4 shows a block diagram of the application of the algorithm in amodulated backlight display. The multi-primary conversion describedhereinbefore is combined with a well known dynamic contrast enhancementtechnique in displays wherein a lighting unit is present to illuminatethe pixel of the display. A well known example is the backlight unitcombined with an LCD.

The block MPA represents the multi-primary conversion under theparticular constraint in accordance with an embodiment of the invention.The multi-primary conversion receives the three input signals R, G, Band supplies the four drive signals P1 to P4. The dynamic contrastenhancement circuit DCE receives the four drive signals P1 to P4 andsupplies corrected drive signals P1′ to P4′ to the four primary displayFPD. The dynamic contrast enhancement circuit DCE further supplies acontrol signal BLM to the backlight unit BL which illuminates the pixelsof the display FPD. The operation of the modulated backlight display iselucidated with respect to FIG. 5.

FIGS. 5A to 5C show graphs for elucidating the application of thealgorithm in the modulated backlight display with three primaries R, G,and B. In all FIGS. 5A to 5C, the horizontal axis represents thepossible values of the input signals R, G, B. In the example shown, theinput signals R, G, B are 8 bit words witch values ranging from 0 to255. The vertical axis represents the number of times a value occurs ina particular input image.

FIG. 5A shows the histogram for the input signal R which usuallyrepresents the red primary color. For the particular input image, thehighest value of the input signal R is MVR. FIG. 5B shows the histogramfor the input signal G which usually represents the green primary color.The highest value of the input signal G is MVG. FIG. 5C shows thehistogram for the input signal B which usually represents the blueprimary color. The highest value of the input signal B is MVB.

In the example shown, the input signal B has the highest maximum value.Consequently, it is possible to amplify the input signals R, G, B allwith a same factor which is selected such that the value MVB becomesequal to 255. The backlight unit BL has to be controlled such that itsluminance decreases with the same factor to obtain a same luminance ofthe display as with the original R, G, B input signals and the originalluminance of the backlight unit BL.

The multi-primary conversion MPA maps the input signals R, G, B (whichusually are the EBU R, G, B signals) into the drive values P1 to P4under the constraint that the maximum value of the drive values P1 to P4is minimal. Consequently, the backlight unit BL can be dimmed maximally.

Alternatively, in other applications, it might be advantageous todetermine the mapping which maps the input signals R, G, B into thedrive values P1 to P4 under the constraint that the minimal value of thedrive values P1 to P4 is maximal to avoid low drive values as much aspossible, for example, to avoid errors in the gamma function of aparticular display.

It is not required that the multi-primary conversion MPA in accordancewith the present invention maps the complete input color range definedby the input signals R, G, B in a same manner into the drive values P1to P4. The multi-primary conversion MPA may use the particularconstraint on a subset of the input color range, for example only thecolors which are near to the boundaries of the image histogram (FIG. 5A-C) defined by the input signals R, G, B.

FIG. 6 shows graphs of the four drive values as a function of the fourthdrive value for an RGBW display.

In a special situation wherein the chromaticity of the W sub-pixelcoincides with the white point of the chromaticity diagram created bythe RGB sub-pixels, the functions F1 to F3 become simpler: all thecoefficients k1 to k3 of Equation 4 have an equal negative value. Thusthe lines representing the functions F1 to F3 intersect the line P4=P4under the same angle. If further the maximal possible luminance of the Wsub-pixel is equal to the maximal possible luminance of theRGB-sub-pixels, then all the coefficients k1 to k3 of Equation 4 havethe value −1, and the lines representing the functions F1 to F3intersect the line P4=P4 under 90 degrees.

In the specific embodiment shown in FIG. 6, the display FPD comprisesRGBW (red, green, blue, white) sub-pixels wherein in equation 4 thecoefficients k1 to k3 are all −1. Consequently, the function F1 to F3are straight lines which intersect the line P4=P4 under 90 degrees andwhich have a start value at the vertical axis of P1′, P2′, and P3′,respectively. Such a display allows for a particular efficient backlightmodulation because most of the colors partly use the white component.The valid range of the fourth drive value P4 now covers the values 0 to1.

The intersection of the functions F1=P1(P4) and F4=P4(P4) occurs forP4=P4 i 13. The intersection of the functions F2=P2(P4) and F4 occursfor P4=P4 i 12. The intersection of the functions F3=P3(P4) and F4occurs for P4=P4 i 11. The values of all functions F1 to F4 isdetermined at each of these intersection values of P4 and at the bordervalues 0 and 1. The maximum values of the functions F1 to F4 at each ofthese values of P4 are compared, and the value of P4 is selected atwhich this maximum value is minimal.

In the example shown in FIG. 6, for P4=0, the maximum value is P1′, forP4=P4 i 11 and for P4=P4 i 12 the maximum value is the value of thefunction F1, for P4=P4 i 13 the maximum value is the value of thefunction F1 which is equal to the value of F4, at P4=p4max, the maximumvalue is the value of the function F4. From FIG. 6, it is clear that theminimum value of the determined maximum values occurs at P4=P4 i 13.

It is important in a system which comprises a camera, a printer, and adisplay, that the correct color is printed and displayed. Therefore, theimage information exchanged between the camera and the printer ordisplay device should be in a universal format. This universal format ispreferably the XYZ color space. The devices which are receiving theimage from the camera have a color management module which converts theimage in the XYZ color space to the color space required by the device.In a printer, this color management module converts the image in the XYZspace usually to a CMY color space. In a display, the color managementmodule converts the image in the XYZ space usually to a RGB color space.However, in a display in accordance with the invention, the colormanagement module in the display converts the image in the XYZ space tothe color space defined by the four primary colors of the foursub-pixels. This conversion may be performed directly or via the RGBcolor space.

Further, it has to be noticed that in practice, the algorithmiccomponents disclosed in this text may be, entirely or in part, realizedas hardware, or as software running on a special digital signalprocessor or a generic processor, etc. The hardware may be a part of anapplication specific IC.

The term “computer program product” should be understood any physicalrealization of a collection of commands enabling a generic or specialpurpose processor to execute any of the characteristic functions of aninvention. The commands may be loaded in one step or in a series ofloading steps into the processor. The series of loading steps mayinclude intermediate conversion steps, such as for example, atranslation into an intermediate language, and/or into a final processorlanguage. In particular, the computer program product may be realized asdata on a carrier such as, for example, a non-transitorycomputer-readable storage medium in the form of a disk or tape, or amemory. The computer program product may alternatively be data travelingover a wired or wireless network connection, or program code on anyother medium such, as for example, paper. Apart from program code,characteristic data required for the program may also be embodied as acomputer program product. Some of the steps required for the operationof the method, such as, for example, data input and output steps, may bealready present in the functionality of the processor instead of beingdefined in the computer program product.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims.

In the claims, any reference signs placed between parentheses shall notbe construed as limiting the claim. Use of the verb “comprise” and itsconjugations does not exclude the presence of elements or steps otherthan those stated in a claim. The article “a” or “an” preceding anelement does not exclude the presence of a plurality of such elements.The invention may be implemented by means of hardware comprising severaldistinct elements, and by means of a suitably programmed computer. Inthe device claim enumerating several means, several of these means maybe embodied by one and the same item of hardware. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

1. A method of converting a three-primary color input signal comprisinga first, a second, and a third input signal (C1, C2, C3) into afour-primary color drive signal comprising a first, second, third, andfourth drive signal (P1, P2, P3, P4) for driving four primary colors ofa multi-primary color additive display (3), the method comprises:defining three functions (F1, F2, F3) representing the first, second,and third drive signal (P1, P2, P3) as a function of the fourth drivesignal (P4), each one of the three functions (F1, F2, F3) being definedby the three-primary color input signal and coefficients defining adependence between the primary colors associated with the first, secondand third drive signals (P1, P2, P3) and the primary color associatedwith the fourth drive signal (P4); determining intersection values (P4i) of the fourth drive signal (P4) at a set of intersections of: thethree functions (F1, F2, F3) mutually, and of the three functions (F1,F2, F3) and a line (F4) defined by the fourth drive signal (P4) beingequal to itself, wherein only the intersection values (P4 i) offunctions having opposite signs of their first derivative are relevant;determining boundary values (P4min, P4max) of a valid range (VR) ofvalues of the fourth drive signal (P4), in which valid range (VR) alldrive signals (P1, P2, P3, P4) have valid values; determining calculatedvalues (CV1, CV2, CV3) of the first, second and third drive signals (P1,P2, P3) at the intersection values (P4 i) and the boundary values(P4min, P4max); selecting a maximum value (Vmax) or minimum value (Vmin)of the calculated values (CV1, CV2, CV3) at each one of the intersectionvalues (P4 i) and each one of the boundary values (P4min, P4max); andselecting the intersection value (P4 i) or the boundary value (P4min,P4max) at which the maximum value (Vmax) or minimum value (Vmin) isminimum or maximum, respectively.
 2. A method as claimed in claim 1,wherein the three functions (F1, F2, F3) are three lines determined by$\begin{bmatrix}{P\; 1} \\{P\; 2} \\{P\; 3}\end{bmatrix} = {\begin{bmatrix}{P\; 1^{\prime}} \\{P\; 2^{\prime}} \\{P\; 3^{\prime}}\end{bmatrix} + {\begin{bmatrix}{k\; 1} \\{k\; 2} \\{k\; 3}\end{bmatrix} \times P\; 4}}$ wherein P1 to P4 are the first, second,third, and fourth drive signal, respectively, P1′, P2′, P3′ are definedby the first, second, and third input signal (C1, C2, C3), and thecoefficients k1 to k3 define a dependence between three primariesassociated with the first, second, and third drive signals (P1, P2, P3),and another primary associated with the fourth drive signal (P4).
 3. Amethod as claimed in claim 2, wherein the fourth drive signal (P4) beingequal to itself is defined byP4=P4′+k4*P4 wherein P4′=0 and k4=1 and wherein for a given pair of thelines of which the associate coefficients (k1, k2, k3, k4) havedifferent signs, the intersection value (P4 i) isP4i=(Pj−Pm)/(km−kj) wherein Pj is the one of the P1′, P2′, P3′, P4′associated with a first line of the pair of lines, Pm is the one of theP1′, P2′, P3′, P4′ associated with a second line of the pair of lines,kj is the one of the coefficients k1, k2, k3, k4 associated with thefirst line, and km is the one of the coefficients k1, k2, k3, k4associated with the second line.
 4. A method as claimed in claim 1,wherein an intersection value (P4 i) outside the valid range of thefourth drive value (P4) is clipped to the one of the boundaries values(P4min, P4max) nearest to the intersection value (P4 i) to obtain aclipped fourth intersection value (P4min; P4max), and wherein theclipped fourth intersection value (P4min; P4max) is used in thedetermining the calculated value (CV1, CV2, CV3, P4 i) associated withthe intersection value (P4 i) outside the valid range.
 5. A method asclaimed in claim 1, wherein the valid range (VR) of the fourth drivesignal (P4) wherein all drive signals (P1, P2, P3, P4) have validvalues, is determined by performing the steps of: determining for thefirst, second, and third drive signal (P1, P2, P3) an respective rangeof the fourth drive signal (P4) for which the first, second, or thirddrive signal (P1, P2, P3) has valid values, and determining (1) thevalid range (VR) as a common range of the fourth drive signal (P4)wherein each one of the first, second, and third drive signal (P1, P2,P3) has valid values.
 6. A method as claimed in claim 5, furthercomprising clipping of the input signal (C1, C2, C3) if the common rangeof the fourth drive signal (P4) is not realizable.
 7. A system forconverting a three primary color input signal comprising a first, asecond, and a third input signal (C1, C2, C3) into a four primary colordrive signal comprising a first, second, third, and fourth drive signal(P1, P2, P3, P4) for driving four primary colors of a multi-primarycolor additive display (3), the system comprises: means for definingthree functions (F1, F2, F3) representing the first, second, and thirddrive signal (P1, P2, P3) as a function of the fourth drive signal (P4),each one of the three functions (F1, F2, F3) being defined by thethree-primary color input signal and coefficients defining a dependencebetween the primary colors associated with the first, second and thirddrive signals (P1, P2, P3) and the primary color associated with thefourth drive signal (P4); means for determining intersection values (P4i) of the fourth drive signal (P4) at a set of intersections of: thethree functions (F1, F2, F3) mutually, and of the three functions (F1,F2, F3) and a line (F4) defined by the fourth drive signal (P4) beingequal to itself, wherein for only the intersection values (P4 i) offunctions having opposite signs of their first derivative are relevant;means for determining boundary values (P4min, P4max) of a valid range(VR) of values of the fourth drive signal (P4), in which valid range(VR) all drive signals (P1, P2, P3, P4) have valid values allowable inthe implementation; means for calculating values (CV1, CV2, CV3) of thefirst, second and third drive signals (P1, P2, P3) at the intersectionvalues (P4 i) and the boundary values (P4min, P4max); means forselecting a maximum value (Vmax) or minimum value (Vmin) of thecalculated values (CV1, CV2, CV3) at each one of the intersection values(P4 i) and each one of the boundary values (P4min, P4max); and means forselecting the intersection value (P4 i) or the boundary value (P4min,P4max) at which the maximum value (Vmax) or minimum value (Vmin) isminimum or maximum, respectively.
 8. A display apparatus comprising thesystem of claim 7, an signal processor for receiving an input signalrepresenting an image to be displayed to supply the first, second, andthird input signal (C1, C2, C3) to the system, and a display device forreceiving the first, second, third, and fourth drive signal (P1, P2, P3,P4) to sub-pixels of the display device.
 9. A display apparatus asclaimed in claim 8, further comprising a light unit for illuminating thesub-pixels and having an input for receiving a modulating signal formodulating the intensity of light generated by the light unit, whereinthe means for selecting a maximum value (Vmax) or minimum value (Vmin)is configured for selecting the maximum value, and the means forselecting the intersection value (P4 i) are configured for selecting theminimum value (Vmin) of the selected maximum values, and wherein thedisplay apparatus further comprises a control unit for receiving thefirst, second, third, and fourth drive signal (P1, P2, P3, P4) to supplythe modulating signal to decrease the intensity of the light and toincrease an amplitude of the first, second, third, and fourth drivesignal (P1, P2, P3, P4) accordingly if the one of the first, second,third, and fourth drive signal (P1, P2, P3, P4) having the largestamplitude has an amplitude lower than or equal to a maximum allowableamplitude.
 10. A display apparatus as claimed in claim 8, wherein thedisplay device has a gamma function, and wherein the means for selectinga maximum value (Vmax) or minimum value (Vmin) is configured forselecting the minimum value, and the means for selecting theintersection value (P4 i) are configured for selecting the maximum value(Vmax) of the selected minimum values.
 11. A display apparatus asclaimed in claim 8, wherein the display device has a gamma function, andwherein the means for selecting a maximum value (Vmax) or minimum value(Vmin) is configured for selecting the maximum value, and the means forselecting the intersection value (P4 i) are configured for selecting theminimum value (Vmin) of the selected maximum values.
 12. Anon-transitory computer-readable storage medium having stored thereon acomputer program comprising processor readable code for causing aprocessor to execute the method of claim 1, the processor readable codecomprising: code for defining three functions (F1, F2, F3) representingthe first, second, and third drive signal (P1, P2, P3) as a function ofthe fourth drive signal (P4), each one of the three functions (F1, F2,F3) being defined by the three-primary color input signal andcoefficients defining a dependence between the primary colors associatedwith the first, second and third drive signals (P1, P2, P3) and theprimary color associated with the fourth drive signal (P4); code fordetermining intersection values (P4 i) of the fourth drive signal (P4)at a set of intersections of: pairs of the three functions (F1, F2, F3)mutually, and of the three functions (F1, F2, F3) and a line (F4)defined by the fourth drive signal (P4) being equal to itself, whereinonly the intersection values (P4 i) of functions having opposite signsof their first derivative are relevant; code for determining boundaryvalues (P4min, P4max) of a valid range (VR) of values of the fourthdrive signal (P4), in which valid range (VR) all drive signals (P1, P2,P3, P4) have valid values allowable in the implementation; code fordetermining calculated values (CV1, CV2, CV3) of the first, second andthird drive signals (P1, P2, P3) at the intersection values (P4 i) andthe boundary values (P4min, P4max); code for selecting a maximum value(Vmax) or minimum value (Vmin) of the calculated values (CV1, CV2, CV3)at each one of the intersection values (P4 i) and each one of theboundary values (P4 min, P4max); and code for selecting the intersectionvalue (P4 i) or the boundary value (P4min, P4max) at which the maximumvalue (Vmax) or minimum value (Vmin) is minimum or maximum,respectively.
 13. The non-transitory computer-readable storage medium asclaimed in claim 12, wherein the computer program is a software plug-infor an image processing application.
 14. A camera comprising the systemof claim 7, and an image sensor supplying the first, second, and thirdinput signal (C1, C2, C3).
 15. A portable device comprising the cameraof claim 14.